The present invention relates to the field of injecting fuel into turbomachines, and it relates more particularly to a system for protecting against excess speed.
In turbomachines, best performance is obtained at speeds of rotation of the rotary members that are close to the maximum acceptable limits, beyond which a compressor or turbine blade might be lost. Unfortunately, such a loss is likely to perforate the engine casing with consequences that can be as bad as complete destruction of the turbomachine.
Thus, and as recommended by international regulations concerning civil aviation, it is essential to use a system to provide protection against excess speeds so as to ensure that the speed of said rotary members does not exceed such authorized maximum speeds.
Until recently, these protection systems made use of regulators of the same type as the Watt fly-weight governor, as shown in international application No. WO 89/02980. However, nowadays, such mechanical regulators are being replaced progressively by electronic regulators which act directly on the engine by interrupting the fuel supply to the combustion chamber of the turbomachine. An example of one such present-day architecture is described in international application No. WO 99/00585.
Unfortunately, that solution suffers from the severe drawback of requiring a restart procedure to be engaged subsequently which, under certain particular conditions of flight, e.g. takeoff, can turn out to be impossible to implement and can lead inevitably to loss of the aircraft.
The present invention thus proposes mitigating those drawbacks by means of an electronic system for providing protection against excess speeds which does not lead to the engine being completely stopped, and which also guarantees a minimum operating speed for maintaining all of the equipment of the aircraft. Another object of the invention is to enable the pilot to act directly on the system in order to unlock it and thus restore the engine to operability.
These objects are achieved by a protection system for providing a turbomachine with protection against excess speed, in which machine the speed of rotation is controlled by a throttle control lever acting via an electronic control circuit, the system comprising means for limiting the fuel feed of said turbomachine to a predetermined fixed flow rate Wps when said speed of rotation exceeds an authorized maximum speed, means for maintaining this fuel feed at said predetermined fixed flow rate while said speed of rotation drops below said authorized maximum speed, and means for returning said fuel speed to a flow rate Wr corresponding to an idling speed when said throttle control lever issues an idling command to said turbomachine via said electronic control circuit.
With the present invention, in the event of excess speed, fuel feed to the engine is reduced to a predetermined fixed fuel feed rate Wps and it is held at this feed rate value until the speed of rotation of the engine slows to less than a predetermined speed, protection subsequently being lifted by the pilot of the aircraft returning the throttle lever to a position Wr for causing the engine to idle.
Preferably, the means for limiting the fuel feed to a predetermined fixed flow rate Wps comprise an electronic circuit for detecting excess speed and producing an electrical signal (I) whenever the speed of rotation (N) of the turbomachine exceeds an authorized maximum speed (N_LIMIT), an excess speed solenoid valve connected to said electronic circuit for detecting excess speed and delivering a first hydraulic signal (Px1) under the control of said electrical signal, and an excess speed valve whose position is controlled by said first hydraulic signal to act by means of a metering valve to which it is connected so as to cause the flow rate of fuel injected into said turbomachine to decrease until the rate reaches said predetermined fixed rate Wps. Depending on the embodiment used, said predetermined fixed flow rate Wps is delivered either by a utilization outlet of said metering valve or by a utilization orifice of said excess speed valve.
The first hydraulic signal corresponds to a low pressure Pb when said speed of rotation of the turbomachine is less than said authorized maximum speed, and to a higher pressure Psf when said speed of rotation (N) exceeds said authorized maximum speed (N_LIMIT). Preferably, said higher pressure Psf is equal to a high pressure P1 available at the outlet from a high pressure fuel injection pump.
In an advantageous embodiment, said predetermined fixed flow rate is maintained while said speed of rotation of the turbomachine drops below said authorized maximum speed by means of a second hydraulic signal (Px2) set to said higher pressure Psf and added to said first hydraulic signal so as to move a hydraulic slider of said excess speed valve against a spring. Said second hydraulic signal is obtained from a hydraulic control signal (Px0) generated via said metering valve from said higher pressure Psf.
The present invention also relates to the method implemented in the above-specified system for providing protection against excess speeds.